leaving a diesel sit for months on end generally isn't an issue other than if you leave the battery connected it might go dead on newer rigs that have onboard computers.
your biggest issue will be if the fuel starts growing algae. if you will be leaving it sit for 6+ months, I would add some additive to keep anything growing in the tank. it usually doesn't take much. with this taken care of, it can sit for a couple of years without much happening other than the usual rotting of the rubber hoses and belts or tires! If there is no antifreeze in the cooling system, drain it so when old man winter comes around you don't have to worry about blown freeze plugs or worse, a cracked block!